1. Field of the Invention
The present invention relates to a supply system for supplying fuels to a fuel cell.
2. Description of the Related Art
Conventionally, a solid polymer membrane-type fuel cell comprises a stack (hereinafter, called a fuel cell) constituted by a plurality of cells, wherein each cell is formed by inserting a solid polymer membrane between an anode and a cathode. Hydrogen is supplied to the anode as a fuel and air is supplied to the cathode as an oxidizing agent, wherein hydrogen ions generated at the anode is moved to the cathode through the solid polymer membrane and electric power is generated by a chemical reaction taking place at the cathode between the hydrogen ions and oxygen.
In order to preserve the ionic conductivity of the solid polymer membrane, an excess water content is added to the hydrogen to be supplied to the fuel cell using a humidification device. In order to prevent a gas passage from clogging due to water accumulating in the gas passage in the electrode, the exhaust fuel is set to a predetermined exhaust flow rate.
Recirculation of the exhaust fuel (hereinafter, this exhaust fuel is sometimes called reflux hydrogen) with the original fuel (hydrogen) makes it possible to increase the fuel efficiency, which results in increased energy efficiency of the solid polymer-type fuel cell.
A conventional example of such a type of fuel cell device is disclosed in, for example, Japanese Unexamined Patent Application, First publication No. Hei 9-213353. In the fuel cell device disclosed in the above-described publication, recirculation of the fuel is carried out by an ejector.
Below, the structure of an ejector is explained. As shown in FIG. 7, the conventional ejector comprises a reflux chamber 2 at the base end of the diffuser 1 which is in a form of a flared pipe, a reflux passage 3 communicated with the reflux chamber 2, and a nozzle 4, which is disposed on the same axial line as that of the diffuser and which is protruded into the diffuser 1. When the fuel to be supplied to the fuel cell is ejected from the nozzle 4 towards the diffuser, a negative pressure is generated at the throat portion 5 of the diffuser 1, the negative pressure draws hydrogen introduced into the reflux chamber 2 to form reflux hydrogen, and the reflux hydrogen is mixed with the hydrogen ejected from the nozzle 4 and the mixture is sent out from the outlet of the diffuser 1.
There is an index called a stoichiometric ratio, which represents the suction efficiency of the ejector. The stoichiometric ratio is defined as a ratio Qt/Qa between Qa and Qt, wherein Qa is a flow rate ejected from the nozzle 4 (that is, the consumed hydrogen flow rate) and Qt is a total flow rate discharged from the diffuser 1. When the flow rate of the reflux hydrogen is assumed to be Qb, since the total flow rate Qt=Qa+Qb, the stoichiometric ratio or the stoichiometric value is defined as Qa+Qb/Qa. When the stoichiometric value is defined as shown above, it is possible to say that the suction efficiency of the ejector increases as the stoichiometeric ratio increases.
In a conventional ejector, since the diameter of the diffuser and the diameter of the nozzle for a diffuser are fixed, a diffuser is typically selected which satisfies a required range of flow rate of the fuel to be used.
FIG. 8 is a diagram, obtained by experiments, showing an example of the relationships between the stoichiometric value and the hydrogen supplying amount Qa (hereinafter, this relationship is called xe2x80x9cstoichiometric characteristicsxe2x80x9d) using the nozzle diameter as a parameter for a fuel supply ejector of a fuel cell device. As shown in FIG. 8, although the increasing stoichiometric value is obtained as the nozzle diameter decreases, the hydrogen flow rate Qa decreases. In contrast, although it is possible to increase the hydrogen flow rate Qa by increasing the nozzle diameter, the stoichiometric value decreases.
As shown by a bold line in FIG. 8, a required stoichiometric value (hereinafter, called xe2x80x9ca required stoichiometric valuexe2x80x9d) for a fuel cell is determined depending on its driving conditions and the flow rates of hydrogen from the idling state to the full open output state changes by 10 to 20 times. Accordingly, it is not possible for one ejector to cover all of the required stoichiometric values.
In order to solve the above-described problem, it is possible to assume an ejector system which is provided with a first ejector for a large flow rate and a second ejector for a small flow rate, and to operate this ejector system such that the fuel is supplied normally using the second ejector for a small flow rate while maintaining the fuel passage to the second ejector in an opening state and when a higher flow rate higher than that of the second ejector is required, the first ejector is operated by opening a magnetic valve disposed for supplying the fuel to the first ejector so that the fuel is supplied by both first and second ejectors.
However, when the above-described ejector system is adopted and when the fuel is supplied using both first and second ejectors, the total aperture area of diffusers of both ejectors becomes too large for the amount of flow to be ejected from the nozzles of both ejectors, and the nozzle size and the optimum value of the diffuser becomes unbalanced, so that the stoichiometric characteristics cannot be satisfied at the time of high flow rate.
An object of the present invention is to provide a fuel supply device capable of preserving the predetermined stoichiometric characteristics over a wide range of flow rate.
According to the first aspect of the present invention, a fuel supply device (for example, an ejector unit 30 in the embodiment described below) for a fuel cell (for example, a fuel cell 11 shown in the embodiment described below) comprising: a plurality of ejectors (for example, a first ejector 40 or a second ejector 50 in the embodiment described below), each comprising a nozzle (for example, a nozzle 41 or a nozzle 51 in the embodiment described below) connected with a fuel passage for ejecting a first fuel (for example, hydrogen in the embodiment described below) and a diffuser (for example, a diffuser passage 43 or a diffuser passage 53 in the embodiment described below) which draws a second fuel (for example, reflux hydrogen in the embodiment described below) by a negative pressure generated by the ejection of the first fuel along the axis direction of the nozzle, for supplying the second fuel by merging with the first fuel; an ejector switching device (for example, a switching valve 60 in the embodiment described below) constituted so as to be able to select and switch any one of the fuel passages of the nozzle among the plurality of ejectors and a housing (for example, a unit body 33 in the embodiment described below) which includes the plurality of ejectors and the ejector switching device.
By constituting the fuel supply device as shown above, it is possible to select any one of the ejectors separately, and by setting different nozzle diameters and different ejector diameters for each ejector, the stoichiometric value may be changed in response to the fuel consumption. Here, the stoichiometric value means a ratio of an amount of a first fuel to the sum amount of the first fuel and a second fuel (that is, the total amount). In addition, since the housing includes a plurality of ejectors and an ejector switching device, the fuel supply device can be made compact.
According to the second aspect of the present invention, in the above fuel supply device for a fuel cell, the housing comprises a first fuel passage through which flows a portion of a first fuel in addition to the first fuel supplied to the nozzles of the plurality of ejectors, and said plurality of ejectors delivers fuel to said first fuel passage.
By the above constitution, the first fuel supplied from the ejector and a portion of the first fuel from the first fuel passage are merged in the first fuel passage and this merged fuel is delivered downstream.
According to the third and fourth aspects of the present invention, in the above fuel supply device for a fuel cell, the fuel supply device further comprises a control device for controlling the ejector switching device in response to an input signal corresponding to a required amount of flow.
By constituting the fuel supply device for a fuel cell as described above, it is possible to select and operate an ejector which is appropriate for the required amount of fuel flow.